The present invention relates to a continuous weighing meter for bulk materials with zero adjustment control.
Several processes and devices for the monitoring and recalibration of the zero point of conveyor scales are known. WO 91/14927 (D1) and WO 95/29390 (D2) may be cited as representing the large number of descriptions, which are part of the state of the technology. The aim of the aforementioned processes and devices and also of the present invention is to increase the accuracy of the determination of the flow of material by a continuous weighing meter. In WO 91/14927, the supply of bulk material to be measured is periodically interrupted to determine the zero point of the weighing returned by the empty belt, while in WO 95/29390, two weighing stations are provided downstream from the despatching station and, within certain tolerances, the two weighing results are applied to form an average.
Both in GB 358 786A and also in FR 2 129 807A, methods and devices are disclosed, with which the belt is weighed twice, indeed with a first weighing device on its upper side together with the bulk material placed on it, and with a second weighing device on its returning underside. In GB 358 786A, purely mechanical means are provided to form the difference between the two weights.
The disadvantage of this method and the corresponding devices lies first in that any material adhering to the belt distorts the second weighing on its underside. Furthermore, the weighing results are not correct if bulk material first adheres to the returning part of the belt, but then in the passage between the guide roller and the second weighing however is not present, or if bulk material falls from the upper side of the belt onto the returning part. Apart from this, in accordance with these publications, neither the position nor the speed of the belt is known. The speed of the belt can therefore be changed in a non-suitable manner, for example as a function of the difference between the target and actual values of the flow of the material, which makes both the maintenance of a desired flow of material and the use of spliced or non-homogeneous belts more difficult or entirely impossible. Furthermore, slip in the belt can neither be established nor compensated using the methods presented above.
In U.S. Pat. No. 2,997,205, a method and a device are presented, with which the belt is weighed twice on its upper side, in a first weighing device without the bulk material placed upon it, and then in a second weighing device together with the bulk material placed upon it. Here also, however, the exact time of running of a determined point on the belt between the two weighing devices is neither known nor of significance.
In none of the devices or methods mentioned above is the exact distance between the two weighing devices, measured along the direction of travel of the belt, of significance.
The periodic interruption of the supply obviously permits the calculation of the possibly varying zero point. This is however troublesome for many applications, since the despatch to another processing station of the material being weighed cannot be interrupted repeatedly without consequence. Added to this, the last calculated zero point remains stored for the period between two such interruptions. In the case of problematical bulk materials such as chocolate solids, flour and other partly sticky substances, the zero point can vary relatively quickly. The process described in WO 95/29390 does not contribute anything to the absolute accuracy of the weighing when residues of the materials mentionedxe2x80x94possibly even on the underside of the conveyor beltxe2x80x94build up slowly and remain there.
The aim which is to be addressed by the present invention, is the production of a continuous weighing meter, with which the zero point can be continuously and permanently determined and is always up to date and available for the processing of the gross weighing.
A continuous weighing meter for bulk material, constructed in accordance with the present invention, includes bulk material input means for supplying bulk material, a conveyor belt onto which the bulk material is deposited, and means for moving the conveyor belt. This continuous weighing meter also includes first weighing means positioned downstream from the bulk material input means and having a first force measuring cell over which the conveyor belt passes and first evaluation equipment for measuring the gross loading of the conveyor belt. This continuous weighing meter further includes second weighing means positioned upstream from the bulk material input means and having a second force measuring cell over which the conveyor belt passes and second evaluation equipment for measuring the tare loading of the empty conveyor belt. Also included in this continuous weighing meter are means for determining the running speed of the conveyor belt and a computer having a central processing unit and computing and control programs and responsive to the gross loading measurement of the conveyor belt, the tare loading measurement of the empty conveyor belt, and the determination of the running speed of the conveyor belt for determining the bulk material load on the conveyor belt during a period of time determined by the distance between the first force measuring cell and the second force measuring cell divided by the running speed of the conveyor belt.
The invention will be described in conjunction with the enclosed drawings.